Speaking to Al Jazeera from the capital Monrovia on Saturday, Tubman said the CDC came to the decision because the party "does not feel that we will be treated fairly in the run-off if we are still using mechanisms and procedures that we used in the first one".

"All we are asking for is that we be allowed to have a closer involvement and being able to monitor and watch the process in the run-off and those requests have not been granted and becuase of that our people have decided that we should not participate in the run-off," he said.

"In order to have democracy in Liberia, we need opposition parties to exist and be viable. And the way this whole process has gone on, our party feels that we are not getting a fair hearing and a fair chance."

The US state department on Saturday said it was disappointed by the CDC's decision and encouraged Liberians toparticipate in the run-off election.

"The CDC's charge that the first-round election was fraudulent is unsubstantiated," state department spokeswoman Victoria Nuland said in a statement.

Leadership change

Tubman previously threatened to withdraw from the run-off unless there was a change of leadership at the election commission.

His party complained of fraud and irregularities, saying the commission was biased. The allegations led to the resignation of election body's director, James Fromayan, last week.

"I chose to step down for the sake of Liberia and so that [Tubman's] CDC would not have an excuse not to participate in the run-off," Fromayan told the Reuters news agency on Sunday.

Fromayan, who denied any wrongdoing, was replaced by his deputy Elizabeth Nelson. He said he did not know if it would be a permanent arrangement.

"Nothing will stop the elections from going ahead as planned," Nelson told Reuters on Friday.

In the first round of voting on October 11, Sirleaf won 43.9 per cent of the vote. Tubman, her closest rival, won 32.7 per cent.

Sirleaf, who won this year's Nobel Peace Prize along with compatriot Leymah Gbowee and Tawakul Karman of Yemen, is a strong favourite for the run-off, having secured the backing of a former rebel leader, Prince Johnson, who was third in the earlier poll with about 11.6 per cent.

Sirleaf was Africa's first democratically elected female president. She is viewed abroad as a Harvard-trained reformer, but Tubman's camp portrays her as out of touch with the impoverished population.